In dense wavelength division multiplexed (DWDM) systems, a pilot tone (PT) is used to conduct optical performance monitoring. The pilot tone is a small and low-frequency modulation (e.g. kHz to MHz) applied to a high-speed optical channel, thus providing an in-band ancillary channel for performance monitoring. The pilot tone may be used to carry wavelength and other link-characterizing information for a fiber link in the DWDM systems.